The present invention generally relates to a cesium oscillator, and particularly to a cesium oscillator capable of providing a high-precision oscillation output based on the atomic resonance effect of cesium.
Currently, a cesium oscillator is used as a high-precision oscillation source in communication systems, for example. A cesium oscillator uses the inherent stability of the atomic resonance frequency of Cs.sup.133.
Referring to FIG. 1, there is illustrated a conventional cesium oscillator. As shown, a conventional cesium oscillator includes a crystal oscillator 411, a modulator 413, an oscillator 415, a frequency synthesiser and multiplier 417, a cesium beam tube 419, a first bandpass amplifier 421, a second bandpass amplifier 423, a lock circuit 425, a synchronous detector 427, and an integrator 429.
The oscillator 411 outputs a signal of a predetermined frequency (10 MHz, for example) to be provided by a cesium oscillator. The cesium beam tube 419 includes a cesium beam source, a cavity, a detector and magnets. The cavity provides a cesium beam derived from the cesium beam source with a microwave of a frequency nearly equal to a predetermined frequency (about 9.192 GHz). The detector detects atoms of Cs.sup.133.
FIG.2 is a graph of a resonance pattern observed in the cesium beam tube 419. In FIG. 2, the horizontal axis represents frequencies of the microwave, and the vertical axis represents the intensity of the observed resonance. It is noted that the frequency of a microwave which actually causes a resonance in the cesium beam tube 419, is equal to the frequency of a signal obtained by phase-modulating the output signal of the crystal oscillator 411 by an output signal of the oscillator 415 (100 Hz, for example) by the modulator 413 and by frequency-synthesizing the phase-modulated signals. Thus, an output signal of the cesium beam tube 419 related to a certain phase-modulated microwave includes an alternating component. The intensity of resonance is proportional to the amplitude level of the above-mentioned alternating component. As a result, the 180.degree. phase inversion occurs in the output signal of the cesium beam tube around peak points (a, b, c in FIG. 2) of the spectrum.
The cesium oscillator controls the frequency of the signal derived from the crystal oscillator 411 based on the alternating output signal extracted from the cesium beam tube 419. A fundamental wave signal included in the output signal of the cesium beam tube 419 is amplified through the first bandpass amplifier 421. Then the amplified signal from the first bandpass amplifier 421 is subjected to a synchronous detection by the synchronous detector 427, which outputs a detection signal. The integrator 429 integrates the detection signal supplied from the synchronous detector 427, and supplies a control terminal 411a of the crystal oscillator 411 with a control voltage based on the integrated output levels derived from the integrator 429. The frequency of the crystal oscillator 411 is controlled based on the applied voltage. In the above-mentioned manner, a feedback loop responsive to the output signal of the crystal oscillator 411 is formed, and thus it becomes possible to fix the output frequency of the crystal oscillator 411 with high precision.
After starting up the cesium oscillator, a second-harmonic frequency signal included in the output signal of the cesium beam tube 419 is amplified through the second bandpass amplifier 423. The second-harmonic frequency signal is a signal of a frequency which is twice the modulation signal, and is generated when a microwave presenting a peak point such as peak point a, b, c, d or e is supplied to the cesium beam tube 419. The phase of the output signal derived from the cesium beam tube 419 is different by 180.degree. on both sides of each of the peak points a, b and c. The lock circuit 425 holds the signal from the second bandpass amplifier 423 when any one of the peaks is detected. Then, the lock circuit 425 supplies the integrator 429 with an instruction based on the peak detection. Then, the feedback loop for controlling the crystal oscillator 411 is established in response to the instruction.
However, the above-mentioned conventional cesium oscillator has the following disadvantages.
First, the crystal oscillator 411 exhibits a frequency transient response as shown in FIG. 3, immediately after starting the power supply to the cesium oscillator. In FIG. 3, the horizontal axis represents time, and the vertical axis represents a variation of frequency (.DELTA.f/f). It is therefore necessary to set the adjustable frequency range (control range) of the crystal oscillator 411 so that the desired (highest) peak point b of the spectrum is present in the adjustable frequency range after the frequency is settled. As a result, the adjustable frequency range of the crystal oscillator 411 must be set narrow, and therefore the gain of the feedback loop is small.
FIG. 4 illustrates an appropriate adjustable frequency range A in which only the peak point b is included. If the adjustable frequency range of the crystal oscillator 411 is wide as indicated by a reference B, an increased gain of the feedback loop can be obtained. However, in this case, there is a possibility that the feedback loop related to the oscillator 411 may be stabilized at the peak point a or c. Further, if the adjustable frequency range deviates from the appropriate position, as shown by a broken line given a reference A,, it is impossible to stabilize the crystal oscillator 411 at the peak point b.
Secondly, the adjustable frequency range shifts due to aging. For example, the adjustable frequency range A shown in FIG. 4 may be changed to the adjustable frequency range A, due to aging. In such cases, the highest-peak point b may be out of the adjustable frequency range. Alternatively, another smaller peak point may be included in the adjustable frequency range in place of or together with the highest-peak point b. In order to avoid this problem, it is necessary to use a high-grade and expensive crystal oscillator. Further, the adjustment procedure is manually performed and is therefore troublesome.